1. Field of the Invention
The present invention relates to a dopant film used as an impurity source adapted to diffuse a semiconductor impurity, such as phosphorus, boron, etc., into a semiconductor wafer (substrate), a method of diffusing an impurity into the semiconductor wafer by the use of the dopant film, and a method of manufacturing the semiconductor wafer.
2. Description of the Related Art
To form a diffusion layer, serving as a collector layer of a transistor, in a silicon wafer for a power device, an impurity is first deposited at a high concentration on both of major surfaces of a raw silicon wafer. Next, glass formed over the surfaces is removed, and then a slumping process is conducted to diffuse the impurity deep into the wafer by a long thermal process at a high temperature, forming impurity diffusion layers over the major surfaces of the wafer. The impurity layer on one major surface of the wafer is subsequently machined away, and then the major surface of the wafer is polished to a mirror surface. The impurity layer over the other surface of the wafer is used as the collector layer of a power transistor. As described above, to diffuse an impurity into a silicon wafer, the impurity layer is deposited over a surface of the wafer, and subsequently diffused into the wafer by long heat treatment of the wafer at a high temperature. To deposit the impurity, there are several methods: (a) coating the surface of the wafer with a liquid impurity by the use of a spin or spray; (b) gasifying an impurity to adhere to the surface of the wafer; and (c) evaporating a solid impurity to adhere to the surface of the wafer.
Recently, a sophisticated method is frequently used which sandwiches a dopant film containing a semiconductor impurity at a high concentration between wafers and directly diffuses the impurity into the wafers from the dopant film. The advantage of the method is to reduce the conventional processes of: depositing an impurity onto a surface of a raw wafer; removing glass formed over the surface of the wafer; stacking the wafer; slumping for diffusing the deposited impurity deep into the wafer; and removing glass formed over the wafer to the processes of: alternate stacking of a wafer and a dopant film; slumping; and removing glass.
As disclosed in U.S. Pat. No. 3,971,870 and Japanese published Patent Specification No. 59-32054, there are two types of dopant film. The first type consists of an impurity compound for diffusion and one selected from the group consisting of cyanoethyl cellulose, methyl cellulose, poly vinyl alcohol, starch and poly vinyl butyral. The second type consists of a vinyl synthetic resin which is an organic binder, an inorganic binder, a proper amount of release auxiliary agent and an impurity compound for diffusion. The vinyl synthetic resin may be a polymer such as poly vinyl acetate, poly methyl vinyl ketone, poly vinyl pyrolidone, methyl acrylate or ethyl acrylate or a copolymer mixed with cellulose nitrate. The inorganic binder may be, in the case of silanols, silicon hydroxide, trimethylene silanol or triethylene silanol, or, in the case of the organic aluminum compound, aluminum methylate, aluminum ethylate, aluminum propylate or aluminum butylate.
Particularly where the second type of dopant film is used, as disclosed in Japanese laid-open Paten Application No. 54-84474, after the organic binder is decomposited and burned at about 500.degree. C., the impurity can be diffused for hours at about 1200.degree. C.
500 to 1,000 wafers are subjected to the impurity diffusion in a lot. Hence, prior to the impurity diffusion a work must be done to take out wafers from a container one by one and sandwich a dopant film between adjacent ones of the wafers. This work is very laborious and time-consuming. Moreover, because of this hand work, inadequate adhesion can occur between the wafer and the film. In the event of the inadequate adhesion, the depth of diffusion of the impurity will vary from wafer to wafer. To eliminate the variation in the diffusion depth, the dopant films must be adhered to the wafers in the same condition. In practice, however, this is impossible because of the hand work. In addition, the wafer and the film might be adhered by undue force, breaking the wafer.
With large-diameter (e.g. 125 mm) wafers for power devices, they are liable to suffer from breakage or a trouble of mask alignment incapability due to the warp of the wafers resulting from the formation of various layers over the wafers. For this reason the wafers are made large in thickness. That is, a power-device wafer has a non-diffusion layer into which various regions are to be formed on a high-concentration diffusion layer (N+ layer or P+ layer) adjacent to a collector layer. Since various processes are needed to form the regions, the wafer must have a minimum thickness to endure these processes. The thickness of the wafer depends on the thickness of the high-concentration impurity layer. The minimum thickness of the high-concentration impurity layer has increased with an increase in the wafer diameter. The high-concentration impurity layer is usually formed by diffusion of impurity into the wafer.
In general, with the diffusion of an impurity into a silicon wafer, the deeper the diffusion, the lower the impurity concentration and the slower the diffusion speed. To diffuse the impurity deep into the wafer, therefore, a heat treatment must be done for hours. However, because of the long heat treatment, the concentration gradient (concentration profile) becomes gentle, increasing the sheet resistance of a high-concentration layer. This will increase the thermal lost at a collector electrode and lower characteristics of the device. To restrict the sheet resistance below a certain value, therefore, the thickness of the high-concentration layer adjacent to the collector layer will suffer from a limitation.
Conventionally, a wafer used for a power semiconductor device is fabricated by lapping a starting or raw wafer of about 600 .mu.m in thickness and processing the lapped wafer. The starting wafer is produced by slicing an ingot and lapping the sliced wafer. To produce the starting wafer of 600 .mu.m in thickness, a material is needed which has a thickness of about 1200 .mu.m, twice that of the wafer. In other words, to produce an starting wafer of 600 .mu.m thick, a material of 600 .mu.m thickness will be wasted.
FIGS. 1A through 1D are sectional views of a structure in processes for producing an end wafer according to a conventional method. A starting OSL wafer 10 of 600 .mu.m thickness is prepared (FIG. 1A). This wafer is produced by slicing an ingot and lapping a resultant wafer. In this case, a material of the ingot, which is wasted by the slicing is about 350 .mu.m in thickness, and a material of the sliced wafer, which is wasted by the lapping is about 200 .mu.m in thickness. Hence, the thickness of the portion of the ingot, which is wasted in producing the starting wafer of 600 .mu.m thick is 550 .mu.m. Subsequently, wafer 10 is subjected to impurity diffusion to form high-concentration impurity diffused layers 11 on either side thereof (FIG. 1B). Thereafter, a side of wafer 10 is removed by a grinder and then subjected to lapping to remove one of diffused layers 11 as shown in FIG. 1C. The surface of wafer 10, from which the diffused layer has been removed, is subsequently mirror-polished to form a wafer with a mirror surface (FIG. 1D). In this way an end wafer or OSL (one side lapped) wafer is produced.
The loss of the wafer due to grinding, lapping and mirror-polishing is about 300 .mu.m in thickness. In the conventional method, although the starting wafer has a thickness of 600 .mu.m, the thickness of the OSL wafer is 300 .mu.m, and hence half of the material is lost. When the wafer is 125 mm in diameter, the amount of the loss will be about 16 g. This is inevitable in the conventional method. The cost assignment of OSL wafers on the market is the profit of 10%, the overhead cost of 15% and the direct cost of 75%. Further, the cost of the raw wafer amounts to 56.4% of the direct cost. Namely, the cost of the wafer itself is very high. Therefore, the conventional method, which wastes a large quantity of material in producing an end wafer, cannot lower the cost of the end wafer.
As described above, the problem with the conventional method is that, since a large proportion of a raw wafer is wasted to obtain an end wafer, the cost of the end wafer becomes very high.